September 8, 2010

An extensive literature exists linking global warming to human health and mortality. There is no doubt that atmospheric conditions have a profound effect on human health; outbreaks of pneumonia, influenza and bronchitis have been linked many times to both weather and climate. Pollen concentrations and pollution levels are certainly related to both weather and human health. Furthermore, the impact of heat waves and cold outbreaks on health and mortality are well established. Humidity not only influences health in terms of allowing bodies to cool in hot periods, but on the other side of the coin, dry, cold air leads to excessive dehydration of nasal passages and the upper respiratory tract increasing chances of microbial and viral infection. Precipitation events, particularly large snowfalls, are directly linked to an immediate decline in human health and an increase in mortality. Given the large literature on the subject, it has been fairly easy for alarmists to make claims that changes in weather and climate will result in higher mortality and an overall decrease in human health.

We have addressed the climate change—mortality connection many times in the past, and while scientists certainly exist claiming the impacts will be catastrophic, others come to the opposite conclusion suggesting global warming might improve human health and extend our lifetimes.

One recent article that we recently ran across appeared in Environmental Research dealing with the diurnal temperature range and daily mortality. Nine scientists from China and North Carolina gathered daily mortality and daily maximum and minimum temperature data from Shanghai, China over the period 2001 to 2004. Kan et al. state “Diurnal temperature range (DTR), defined as the difference between maximal and minimal temperatures within 1 day, is another meteorological indicator associated with global climate change and urbanization. In most urban regions of the world, DTR has been decreasing because nocturnal minimum temperatures have risen faster than daytime maximum temperatures. In some areas (e.g. parts of New Zealand and alpine regions of central Europe) maximum and minimum temperatures have increased at similar rates, and thus DTR has remained constant. However, in other areas (e.g. India), DTR has increased as a result of a decrease in the minimum temperature”. They speculate “We hypothesized that large diurnal temperature change might be a source of additional environmental stress, and therefore a risk factor for death.”

Kan et al. used some statistical wizardry to control for day of week, air pollution, mean temperature, and relative humidity. They state “We found a strong association between DTR and daily mortality after adjustment for those potential confounders. A 1ºC increment of the 3-day moving average of DTR corresponded to a 1.37% increase in total non-accidental mortality, a 1.86% increase in cardiovascular mortality, and a 1.29% increase in respiratory mortality.” They warn “In summary, we found that DTR is independently associated with daily mortality in Shanghai. Although the association between climate change and DTR varies across the globe and our results might not necessarily apply to other areas of world, our data suggest that even a slight increase in DTR is associated with a substantial increase in mortality.”

Of course, it would be just as valid to say “our data suggest that even a slight decrease in DTR is associated with a substantial decrease in mortality.” Kan et al. acknowledge that DTR is decreasing in most cities and DTR has generally decreased at the global scale. The latest United Nations IPCC report clearly states “Diurnal temperature range (DTR) decreased by 0.07°C per decade averaged over 1950 to 2004, but had little change from 1979 to 2004, as both maximum and minimum temperatures rose at similar rates.” Not much to get overly alarmed about.

Our second article on climate change and mortality appeared in Occupational and Environmental Medicine a couple of years ago and was written by seven scientists from the United States and the United Kingdom. Carder et al. begin noting “Mortality rates for cardiovascular and respiratory disease typically exhibit distinct seasonal variation with the highest rates occurring in the winter months. For Scotland, the percentage summer to winter difference in weekly all cause mortality rates is estimated to be in the order of 30%. The main factor considered to be influencing the observed seasonal pattern is the relation between mortality and temperature. The association between low temperature and increased morbidity and mortality is well recognised.” To explore the mortality – low temperature connection, the team gathered daily mortality data for Scotland’s three largest cities for the period January 1981 to December 2001 (Scotland’s three largest cities are Glasgow, Edinburgh, and Aberdeen). In addition to other goals, Carder et al. examined whether or not wind chill (which is markedly worsened by high wind speeds) would be a better predictor of mortality than temperature alone.

The article spares the reader all the gory details and includes a “Main messages” section in which Carder et al. write “Cold temperature is a strong predictor of mortality in the Scottish population. The strongest associations were observed between cold temperature and respiratory mortality. The effects of cold temperature on mortality persisted for periods in excess or two weeks. The evidence does not suggest that wind chill temperature, as measured by the Steadman Index, is likely to be a better predictor of mortality than dry bulb temperature.” In a final summary section entitled “Policy implications”, Carder et al. state “Cold temperature is a public health problem. The most significant mortality outcomes are cardiovascular and respiratory although it is open to speculation as to whether people with prior cardiorespiratory disease are more susceptible to cold related mortality.” The root cause of the cold temperature – high mortality connection is suggested as “Cold related increases in respiratory mortality are generally attributed to cross infection from increased indoor crowding during colder months and to the detrimental effect of exposure to cold temperatures on the immune system’s resistance to respiratory infections. The observed relation between cold temperature and mortality was typically stronger among the elderly.”

In work done on weather-related mortality in major U.S. cities, Robert Davis and colleagues (which include several WCR contributors), also found a clear increase in mortality in the winter months (over the average mortality during the summer months). However, Davis et al. found only a very weak association between mortality and temperature during the winter months, and instead suggested that the mortality impact from influenza likely swamped any impact from temperature during this time of the year.

The bottom line though, is that the results from these studies suggest that the ultimate impact of climate change—especially a winter-dominated and/or nighttime-dominated warming—on weather-related mortality is likely minimal at worst, and perhaps, at best, even slightly beneficial.